Methods and compositions for stripping nickel



United States Patent 0 3,351,556 METHODS AND COMPOSITIONS FOR STRIPPZNGNICKEL Harry K. Tsourmas, North Haven, Conn., assignor to MacDermicl,Incorporated, Waterbury, Conu., a corporation of Connecticut No Drawing.Filed Mar. 24, 1965, Ser. No. 442,506 12 Claims. (Cl. 252-102) ABSTRACTOF THE DISCLOSURE Compositions and aqueous solutions thereof forstripping nickel coatings from substrates, particularly where copper orcopper alloys are present in the substrate. The composition comprisesgenerally the combination of an oxidizer consisting of an oxy-halogencompound of the group comprising specifically the chlorites,hypochlorites, bromates and hypobromites; a nickel complexing agent ofthe group comprising ammonia, the ammonium ion, amines and other knownnickel complexers; a divalent sulfur ion-supplying compound, eitherorganic or inorganic, as a nickel activator; and, where necessary toprevent poisoning of the stripping solution by the presence of copper,an organic sulfur ion-supplying compound if the nickel activator is notof such type or present in sufiicient amount.

This invention relates to dissolving nickel from a substrate on which ithas been plated, and is more particularly directed to compositions forand methods of stripping nickel deposits from ferrous metals wherecopper or copper alloys may or may not be present. The invention is alsouseful in stripping nickel from organic, i.e. plastic, or inorganic,i.e. ceramic, substrates.

Fabricated articles incorporating a plating of nickel sometimes fail topass inspection because of defects in the plate. The reclaiming of thebasic fabricated article in many instances is of significant economicimportance. Usually the basic article, quite apart from the defectivenickel deposit, represents considerable value in terms of labor andmaterials, and consequently if this can be salvaged in reusable form byremoving the defective deposit, worthwhile economies can be effected.

There have been many compositions developed for stripping various plateddeposits, including nickel, from their substrates. Some of theseincorporate strongly acidic systems which attack the substrate as wellas the nickel deposit which it is desired to remove. Others incorporatecyanide compounds which introduce health hazards. Still others involvewaste liquor disposal problems. Many of the earlier systems also arerather slow in operation, are critical in respect to control ofoperating conditions and have rather limited capacity for dissolvingnickel before they must be replenished or entirely replaced.

I have discovered improved compositions effective in stripping nickeldeposits in aqueous solution which offer certain desirable features andadvantages over the prior art. My system is completely alkaline and thusespecially adapted to stripping of nickel from ferrous substrates whichare rapidly attacked in acid environments employed in many of thestripping systems used heretofore. My system is operative in thepresence of copper which very commonly is used as a flash or strike fornickel but which acts as a poison in some of the more effective priorstripping compositions, rendering them useless. My system avoids healthhazards inherent in systems employing cyanide compounds used heretofore,being entirely free of cyanides in its make-up. My composition isinherently able to activate the nickel surface to be dissolved and tomaintain a high rate of metal dissolution until the ratedeterminingcomponent or components are consumed. One of the most important featuresof my composition is the high rate of nickel dissolution obtained atlow, ambient temperatures, which drastically reduces evaporation lossesand prolongs the useful life of the stripping system. Another advantageof my system is that it presents no waste liquor disposal problems suchas those encountered with cyanides and chromates, since the Waste may bedischarged as an efiluent without treatment other than dilution if localauthority permits nickel and copper in the effluent. If this is notpermitted, these metals can very readily be precipitated from my system.

The composition is applicable to stripping nickel from the usual ferroussubstrates, including those containing a flash deposit of copper as forexample in automotive hardware or the like. It is also useful, ofcourse, in stripping from non-metallic basis articles such as plasticsand ceramics comonly used in electronic components as well as automotiveapplications.

The stripping compositions herein disclosed consist essentially of anoxidizer, a complexing agent for the nickel, a sulful-containingactivator for the nickel and, where necessary, a deactivating agent forcopper to prevent cop per poisoning of the stripping solution.

The oxidizers in my system are oxy-halogen compounds selected from thegroup consisting of the alkali and alkaline earth metal normalchlorites, hypochlorites, bromates and hypobromites. Sodium or potassiumchlorites are preferred for reasons of availability and cost.

The nickel complexer may be the ammonium ion from aqueous ammonia orbuffered as ammonium carbonate, chloride, fluoride, phosphate, sulfate,nitrate, or acetate salts; amines and substituted affiines such asethylenediamine, the disodium salt of ethylenediamine tetraacetic acid;nitrilotriacetic acid and its salts, and amino acidssuch as glycine andits salts, e.g. sodium glycinate; the salts of glycolic acid, gluconicacid; and other hydroxy acids such as tajarjg The inorganic ammoniumsalts are especially good from a cost standpoint. The amines on theother hand have the added advantage of lower volatilization losses.

As nickel activators, I use sulfur-containing compounds belonging to thegroups comprising metallic sulfides, nonmetallic sulfides, thioacids andtheir salts, aliphatic nitrogen sulfur compounds and aromatic nitrogensulfur compounds. In fact almost any organic or inorganic sulfurcompound having at least limited solubility in aqueous solution iseffective.

Finally, where a deactivator for copper is needed, I prefer to usediethyldithiocarbamic acid sodium salt, sub stituted dithiocarbamates,thioanilids, and organic sulfurcontaining carbamides.

In each of the foregoing categories, mixtures of the agents mentioned,rather than just a single agent, may be employed provided of course themixture does not itself interact to destroy the effectiveness of theagents.

Solutions containing,- the foregoing combination of components areoperative over a plj ar ge pf about 8.0 t o 1 .Q and, where necessary,an amine is added to effect the desired degree of alkalinity withinthese limits. Generally a pH of 8.5 to 10.5 is preferred.

The stripping solution is not critical as to concentration of oxidizer,nickel complexer, activator for the nickel or deactivator for thecopper. Practically useful solutions are obtained when the oxidizerconcentration is as little as 0.1 mol per liter up to near saturation.The complexer concentration is dependent upon the amount of nickel to bedissolved. The system is operative where the complexer is present at amol ratio of at least 1:1 but for practical operation, more usefulstripping speeds are obtained with a mol ratio preferably of 3:1 orgreater. The amount of menu the complexer needed may be determined fromthe amount of the oxidizer required to oxidize the elemental nickeldeposit to be dissolved by converting it to the nickelous state. Theactivator is found to be effective in molar concentrations from traceamounts as little as 0.00023 to 0.23 mol per liter. The preferredconcentration is about 0.0023. The copper deactivator is likewise foundto be effective over a range of concentrations from about 0.008 to 0.2mol per liter, preferably within the limits of solubility, or about 0.01to 0.015 mol per liter. In some cases a single sulfur-containingcompound may serve both as activator for nickel and as deactivator forcopper, in which case the molar concentration should equal the sum ofthe concentrations of the separate components.

The temperature of the solution used in stripping nickel is notcritical, and the system exhibits outstanding improvement over earliersystems at ambient room temperatures, making commercial operationpractical under conditions which minimize evaporation losses and effectrelated advantages of economy and installation simplification.

In this system, in the process of activating the nickel surface by thesulfur-containing compounds, a film forms on the dissolving nickel or onthe substrate. This film is essential for dissolution of the nickel butshould be removed from the substrate before performing further platingor finishing operations. Such film or smut removal may be accomplishedin any of several well known ways, or it can be effected in a solutionof the same oxidizer and complexer, but without a nickel activator, usedin the stripping bath itself.

Examples of typical solutions within the purview of the invention, andthe stripping rates obtained by them,

.are as follows:

Example 1 The following solutions are useful in stripping nickel from asubstrate where no copper is present:

Mol/l Moi/l. LIOl/l.

Ammonia, aqua 4. 50 3. 37 2. 25 Ammonium carbonate 2.25 3.37 4. 50Sodium clilorit.o 0.375 0.375 0.375 Ammonium thiocyanate. 0. 0023 0.0023 0. 0023 Mil/Min. Mil/Min. Mil/M in.

(At 235 C.) (At 25 C.) (At 25 C.)

Nickel stripping speed... 0. 002 0. 058 0. 002

Substitution of any of the anions named hereinabove for carbonate in theammonium complexer is effective, and produces no significant change instripping rate at the same mol concentrations. Likewise substitution maybe made of other cations, such as potassium, lithium or calcium forexample, in the oxidizer, again without change in molar concentration,and will be operative.

Example 2 Where copper is present as a strike under the nickel, thefollowing system is effective:

Mol/l.

Ammonia, aqua 450. Ammonium carbonate 2.25 Sodium chlorite 0.375Ammonium thiocyanate 0.0023 Sodium diethyldithiocarbamate 0.008 to 0.016Nickel stripping speeds,

(mil/min. at 25 C.) 0.060.

Example 3 Compositions identical with those of Examples 1 and 2 exceptfor the substitution of different complexers provide effective strippingalso. For example, in place of the ammonia-ammonium carbonate, equalmolar concentrations of ethylene-diamine may be employed. Providing alsothat the pH of the solution is maintained at the aforesaid level of 8.0to 13.0, where necessary by the addition of aqueous ammonia orethylene-diamine, other complexers such as disodium ethylenediaminetetraacetic acid, nitrilotriacetic acid, glycine or glycolic, gluconicand tartaric acids and their salts may likewise be used. The molarconcentrations of these complexers, either singly or in combination,must of course be sufficient to provide the desired mol ratio of atleast 1:1 and preferably higher in respect to the amount of nickel to bedissolved. One of the major advantages of the foregoing group ofcomplexers is a lower volatility than the straight aqueousammonia-ammonium salt compositions. Stripping rates are generallysatisfactory although generally lower than in Example 1. The system isoperative at a temperature as low as 15 C. but a preferred practicallower limit is around 25 C. for the system of Examples 1 and 2. Othersystems, as noted hereinafter may require somewhat higher temperatureson the order of 50 to 75 C. to provide practical stripping speeds.Examples using other com- Example 4 In place of sodium chlorite inExamples 1 and 2, other oxidizers may also be used. Thus, other chloritesalts such as potassium, lithium or calcium are fully operative whensubstituted in equal molar concentrations. Still other oxidizers of theoxy-halogen group which are effective are the corresponding bromatesalts and the hypobromites. The iodates and fiuorates do not appear tobe operable in this system. Again, typical nickel stripping rates forsystems employing some of these other oxidizers are as follows:

TABLE 2 Moi/l. Mol/l.

Ammonia, aqua 4.0 9.0 Ammonium carbonat 2. 25 4. 45 Sodium hypochlorite0. 375 Sodium hypobromite l 0. 75 Sodium thioeyanate- 0. 0023 0.0023 pH9-10 9-10 Mil/Min. Mil/Min. (At 49-54" C.) (At iii-27 C.)

Nickel stripping 0.002 0.0005

Example 5 Nickel activators other than ammonium thiocyanate as given inExamples 1 and 2 includes such divalent sulfur ion-supplying compoundsas sodium, potassium, ammonium, calcium, barium, magnesium, aluminum andhydrogen sulfides, and carbon disulfide. Organic nitrogen sulfurcompounds, both aliphatic and aromatic, are operative. For example,thioanilids and thiocarbamides, such as dimethylor diethyl-thiourea andthe corresponding dithiocarbamic acids, and their salts, thioacetic acidand thioamide (e.g. thioacetamide) derivatives, are typical of usefulaliphatic compounds. Correspondingly, aromatic nitrogen-sulfur compoundssuch as thiocarbanilide (diphenylthiourea) and phenyldithiocarbamic acidand its salts are operative. Again at the same molar concentrations, thenickel stripping rates remain good with such activators.

TABLE 3 Mol/l. Ammonia, aqua 4.0 Ammonium carbonate 2.25 Sodium chlorite0.375 Thiourea 0.023 Nickel stripping speed (mil/min. at 25 C.), 0.0137.

Example 6 Example 7 For practical purposes, a minimum molarconcentration of the complexer in solution should be about 3:1 relativeto the nickel. With this in mind it is sometimes advantageous to preparea composition in the form of a concentrate for reducing transportationcosts. In such a case, a solution having a make-up composition ofcomplexer, oxidizer and activator mol ratio of around 3.0:0.3:0.003 isquite suitable. Where a combined activator-deactivator is used, theratio should be 3.0:0.3:0.01. Preferably, a higher concentration ofcomplexer in the bath is desirable but this can be conveniently added inthe form of aqueous ammonia, ethylenediamine, etc. at the time of makingup the actual solution to be used in the stripping process. The minimumpH of 8.0 must also be observed, of course.

Example 8 It is also possible to use ethylenediamine tetraacetic (EDTA)both as a nickel complexing agent and as a copper deactivator, but insuch instance a sulfur-type nickel activator must also be present. Whilethe EDTA might itself serve as the only nickel complexer in solution,this increases the expense and therefore it is more practical to use itin combination with ammonia or one of the ammonium salts such as thenitrate or sulfate. The molar concentrations of EDTA, either alone or incombination with an ammonium ion supplying component, should in thiscase equal the total of the molar concentrations recommended above forthe complexer plus the copper deactivator. Since the latter requiresonly trace amounts to be efiective, maintaining the molar conentrationof the complexing agent (EDTA alone or in combination with others named)above the minimums recommended for the complexing agent alone will besufficient.

As discussed above, one of the principal advantages of the systemsherein disclosed is that they are operable in terms of practicalcommercial use where copper ions get into the stripping solution. Thiscan and does occur, for example in the commonly encountered situationwhere there is, in addition to the nickel plate, a flash coating ofcopper on the substrate. This flash coating is on the order of 0.03 milthick in a typical case, and is stripped along with the nickel in mysolution. But the copper thus brought into solution does not poison thestripping bath so long as its concentration remains below a weight ratioof about 1:60 relative to nickel. The amount of copper in the usualfiash coating is well below that needed to reach such a concentration.Thus my solution has the advantage of far higher stripping rates thancyanide and other alkaline solutions, and also avoids the health hazardsof the cyanide systems.

What is claimed is:

1. A nickel stripping composition for use in aqueous solution, whichconsists essentially of:

(a) an oxidizer selected from the group consisting of the sodium,potassium, calcium and lithium salts of hypochlorite, normal chlorite,bromate and hypobromite;

(b) a nickel-complexing agent selected from the group consisting ofammonia and ammonium carbonate, chloride, fluoride, phosphate, sulfate,nitrate and acetate; ethylenediamine, ethlenediamine tetraacetic acid;nitrilotriacetic acid; glycine, glycolic, gluconic and tartaric acidsand salts thereof; and

(c) a divalent sulfur ion-supplying compound as a nickel-activatingagent, said compound being selected from the group consisting of sodium,potassium, calcium, barium, magnesium, aluminum, ammonium, hydrogensulfides and carbon disulfide; thiocyanates, thioacetates, thioamides,thioanilids, thiocarbamides and the salts thereof;

said oxidizer being present in amount to provide in solution from about0.1 mol per liter to near saturation; said complexing agent beingpresent in 8. mol ratio of at least 1:1 relative to the nickel to bedissolved; and said activat ing agent being present in small buteffective amount up to about 0.23 mol per liter.

2. A composition for selectively stripping nickel deposits fromsubstrates on which they are plated, which composition is operative inthe presence of copper ions in the stripping solution and which consistsessentially, in combination with the composition defined in claim 1, ofa copper-deactivating agent selected from the group consisting ofmethyl, ethyl and phenyl substituted thioanilide and dithiocarbamide andthe sodium, potassium and ammonium salts thereof, saidcopper-deactivating agent being present in small but effective amount upto about 0.23 mol per liter.

3. A composition for selectively stripping nickel deposits fromsubstrates on which they are plated, which" composition is operative inthe presence of copper ions in the stripping soluiton and consistsessentially of:

(a) an oxidizer selected from the group consisting of the sodium,potassium, calcium and lithium salts of hypochlorite, normal chlorite,bromate and hypobromite;

(b) a nickel complexing agent selected from the group consisting ofammonia and ammonium carbonate, chloride, fluoride, phosphate, sulfate,nitrate and acetate; ethylenediarnine; ethylcnediamine tetraacetic acid;nitrilotriacetic acid; glycine, glycolic, gluconic and tartaric acids;and

(c) a nickel-activating, copper-deactivating divalent sulfurion-supplying compound selected from the group consisting ofthiocarbanilide, dimethyldithiocarbamic, diethyldithiocarbamic andphenyldithiocarbamic acids and salts thereof;

said oxidizer being sufiicient to provide in solution at least 0.1 molper liter to near saturation, said complexing agent being present in amol ratio of at least 1:1 relative to nickel to be dissolved; and saidnickel-activating, copper-deactivating agent being present in small buteffective amount up to about 0.23 mol per liter.

4. A nickel stripping composition for use in aqueous solution whichconsists essentially of ammonium carbonate, sodium chlorite and ammoniumthiocyanate in mol ratios of approximately 3.0:0.3:0.003.

5. A nickel stripping composition for use in aqueous solution whichconsists essentially of approximately 3 mols of ammonium carbonate, 0.3mol sodium chlorite, and 0.01 mol sodium diethyldithiocarbamate.

6. A composition for use in aqueous solution in dissolving nickeldeposits from substrates on which they are plated, which consistsessentially, in addition to water, of an oxidizer selected from thegroup consisting of the sodium, potassium, calcium and lithium salts ofchlorite,

hypochlorite, bromate and hypobromite; a nickel complexing agentselected from the group consisting of ethylenediamine, disodiumethylenediamine tetra'aicetic acid, nitrilotriacetic acid, glycine,glycolic, gluconic and tartaric acids; and a sulfur-containing nickelactivating agent capable of providing divalent sulfur ions in solution;said oxidizer being present in amount to provide in solution from about0.1 mol to near saturation, said complexiag agent being present in a molratio of at least 1:1 relative to the nickel to be dissolved, and saidnickel activating agent being present in small but effective amount upto about 0.23 mol per liter.

7. A composition for use in aqueous solution for dissolving nickeldeposits from substrates on which they are plated, which consistsessentially, in addition to water, of an oxidizer selected from thegroup consisting of the sodium, potassium, calcium and lithium salts ofhypochlorite, normal chlorite, bromate and hypobromite; a nickelcomplexing agent selected from the group consisting of ethylenediamine,disodium ethylenediamine tetraacetic acid, nitrilotriacetic acid,glycine, glycolic, gluconic and tartaric acids; and a sulfur-containingnickel activating agent capable of providing divalent sulfur ions insolution; said oxidizer being present in amount to provide in solutionfrom about 0.1 mol per liter to near saturation, said complexing agentbeing present in a mol ratio of at least lzl relative to the nickel tobe dissolved, and said activating agent being present in small buteffective amount up to about 0.23 mol per liter.

8. A composition as defined in claim 7, wherein the sulfur-containingnickel activating agent is selected from the group consisting ofsulfide, thiocyanate, thioacetate, thioamide, thioanilid andthiocarbamide.

9. The method of dissolving metallic nickel deposits from articleshaving deposits thereof on their surfaces, which comprises subjectingthe surface of the article to an aqueous solution consistingessentially, in addition to water, of:

(a) an oxidizing agent in amount of from about 0.1 mol per liter to nearsaturation, selected from the group consisting of the sodium potassium,calcium, and lithium salts of hypochlorite, normal chlorite, bromate andhypobromite;

(b) a nickel-complexing agent, in amount providing a mol ratio of atleast 1:1 relative to the nickel to be dissolved, selected from thegroup consisting of ammonia and ammonium carbonate, chloride, fluoride,phosphate, sulfate, nitrate and acetate; ethylenediamine;ethylenediamine tetraacetic acid, nitrilotriacetic acid; glycine,glycolic, gluconic and tartaric acids and salts thereof; and

(c) a divalent sulfur ion-supplying compounds as a nickel activatingagent, in small but effective amount up to about 0.23 mol per liter,selected from the group consisting of sodium, potassium, calcium,barium, magnesium, aluminum, ammonium, hydrogen sulfides and carbondisulfide; thiocyanates, thioacetates, thioamides, thioanilids,thiocarbamides and the salts thereof; while maintaining said solution attemperatures ranging from approximately 15 to C.

10. The method defined in claim 9 wherein copper ions may also bepresent in solution, which comprises subjecting the surface of thearticle to an aqueous solution as defined in claim 9 but also containinga small but effective amount up to about 0.23 mol per liter of a copperdeactivating agent selected from the group consisting of the methyl,ethyl and phenyl substituted thioanilids and dithiocarbamides and thesodium, potassium and ammonium salts thereof.

11. The method defined in claim 9 wherein copper ions may also bepresent in solution, which comprises subjecting the surface of thearticle to an aqueous solution as defined in claim 9, wherein saidnickel activator is present in amount of at least about 0.01 mol perliter and is selected from the group consisting of thiocarbanilide,dimethyldithiocarbamic, diethyldithiocarbamic and phenyldithiocarbamicacids and the sodium, potassium and ammonium salts thereof.

12. The method of dissolving metallic nickel deposits from articleshaving deposits thereof on their surfaces, which comprises subjectingthe surface to an aqueous solution consisting essentially in addition towater of an oxidizer selected from the group consisting of the sodiumpotassium, calcium and lithium salts of chlorite, hypochlorite, bromateand hypobrornite; a nickel complexing agent selected from the groupconsisting of ethylenediamine, disodium ethylenediamine tetraaceticacid, nitrilotriacetic acid, glycine, glycolic, gluconic and tartaricacid; and a sulfur-containing nickel activating agent capable ofproviding divalent sulfur ions in solution; said oxidizer being presentin amount to provide in solution from about 0.1 mol to near saturation,said complexing agent being present in a mol ratio of at least 1:1relative to the nickel to be dissolved, and said nickel activating agentbeing present in small but effective amount up to about 0.23 mol perliter; while maintaining said solution at temperatures ranging fromapproximately 15 to 75 C.

References Cited UNITED STATES PATENTS 2,561,065 7/1951 Meyer 252-103 X3,102,808 9/1963 Weisberg et al. 252364 X 3,163,524 12/1964 Weisberg etal 7597 X

1. A NICKEL STRIPPING COMPOSITION FOR USE IN AQUEOUS SOLUTION, WHICHCONTAINS ESSENTIALLY OF: (A) AN OXIDIZER SELECTED FROM THE GROUPCONSISTING OF THE SODIUM, POTASSIUM, CALCIUM AND LITHIUM SALTS OFHYPOCHLORITE, NORMAL CHLORITE, BROMATE AND HYPOBROMITE; (B) ANICKEL-COMPLEXING AGENT SELECTED FROM THE GROUP CONSISTING OF AMMONIAAND AMMONIUM CARBONATE, CHLORIDE, FLUORIDE, PHOSPHATE, SULFATE, NITRATEAND ACETATE; ETHYLENEDIAMINE, ETHLENEDIAMINE TETRAACETIC ACID;NITRILOTRIACETIC ACID; GLYCINE, GLYCOLIC, GLUCONIC AND TARTARIC ACIDSWITH SALTS THEREOF; AND (C) A DIVALENT SULFUR ION-SUPPLYING COMPOUND ASA NICKEL-ACTIVATING AGENT, SAID COMPOUND BEING SELECTED FROM THE GROUPCONSISTING OF SODIUM, POTASSIUM, CALCIUM BARIUM, MAGNESIUM, ALUMINUMAMMONIUM, HYDROGEN SULFIDES AND CARBON DISULFIDE; THIOCYANATES,THIOACETATES, THIOAMIDES, THIOANLIDS, THIOCARBAMIDES AND THE SALTSTHEREOF; SAID OXIDIZER BEING PRESENT IN AMOUNT TO PROVIDE IN SOLUTIONFROM ABOUT 0.1 MOL PER LITER TO NEAR SATURATION; SAID COMPLEXING AGENTBEING PRESENT IN A MOL RATIO OF AT LEAST 1:1 RELATIVE TO THE NICKEL TOBE DISSOLVED; AND SAID ACTIATING AGENT BEING PRESENT IN SMALL BUTEFFECTIVE AMOUNT UP TO ABOUT 0.23 MOL PER LITER.